Sampling or sample-and-hold circuits are commonly used in semiconductor products. In general, sampling circuits capture and store a voltage input signal and provide a voltage output signal which is equal to or proportional to the input signal. In many applications, a plurality of input signals need to be sampled and held. Others have used a sample-and-hold circuit for each input signal with the disadvantage of having to implement a significant amount of additional circuitry.
Known multiple channel circuits are susceptible to an error commonly referred to as "cross-talk" error. Cross-talk is an interaction between signals or circuits of different channels. In one form, cross-talk is an interaction between a channel which is not in use and a channel which is in use.
A known circuit which helps reduce the cost of multiple sample-and-hold circuits uses a multiplexor circuit in conjunction with a single amplifier. In one form of this technique, a plurality of transistor switches couples an analog input signal to a single storage capacitor which is commonly utilized among multiple input channels. One transistor switch is provided for each separate analog signal and within the sample-and-hold circuit. A performance disadvantage with providing a commonly utilized storage capacitor connected to each transistor switch is the existence of a time delay required to change a voltage associated with each separate analog signal at the commonly utilized storage capacitor. That is, when the voltages of the separate analog input signals are substantially different, parasitic resistances and capacitances associated with input circuits external to the sample-and-hold circuit cause an unwanted time delay. The unwanted time delay limits either the accuracy or the operating speed of this known multichannel sample-and-hold circuit.
Yet another known multiple channel system multiplexes a plurality of channel inputs to a single A/D converter. Many applications require a resistor/capacitor (RC) filter network at the input of each analog input. Frequently, the impedance of such a filter network is greater than ten thousand ohms. A disadvantage with a multiplexed A/D converter is the presence of an error commonly referred to as "cross-talk" between the channels. Cross-talk can be created as a result of the A/D converter having an input coupled to a capacitive node. As a result of the capacitance, there is charge sharing between a previous data sample and a current data sample. The charge sharing causes a previously selected input signal to affect a selected input signal's value. It is desired to operate the system at higher frequencies, but because the time constant of the RC filter network is significant, there is not enough time to correct the charge sharing error between two successive data samples. Therefore, unless enough time is allowed for the capacitive input node to charge completely, cross-talk errors are usually present.
In a multiple channel circuit, if a semiconductor junction of an input protection structure or an analog switch which is not in use is forward biased to be substantially conductive, charge carriers will be injected into an underlying semiconductor region of the circuit. If these carriers are "collected" by the semiconductor junctions of the circuitry of a channel in use, the carriers which comprise a current will produce a voltage difference across the impedance of the circuitry of that channel. If this voltage difference is present when a channel is sampling, the voltage difference will produce cross-talk error in the sample.